1. Field of the Invention
The present invention relates to a method of non-destructively measuring a hole diameter, a hole position, a hole surface roughness, or a bending loss of a holey optical fiber, a manufacturing method of a holey optical fiber employing the measuring method, and a test method of an installed optical line having a holey optical fiber.
Priorities are claimed on Japanese Patent Application No. 2010-185319, filed Aug. 20, 2010, and Japanese Patent Application No. 2011-021886, filed Feb. 3, 2011, the contents of which are incorporated herein by reference.
2. Description of Related Art
A holey optical fiber has a structure in which a plurality of holes is formed in an optical fiber made of quartz glass or the like, and these holes are extended along the longitudinal direction of the optical fiber. The holey optical fiber can obtain optical properties that cannot be realized in conventional optical fibers due to the presence of the holes.
For example, as reported in Japanese Patent No. 3854627 and K. Nakajima et al., 58th IWCS, 9-3, 2009, in a hole assist optical fiber in which a plurality of holes is arranged around a core with a high refractive index, marked improvements of bending loss characteristics are expected due to the increase of an effective refractive index difference by the holes.
Such a holey optical fiber is obtained by drawing a holey optical fiber preform, for example, as described in Japanese Unexamined Patent Application, First Publication No. 2002-145634 or the like. Additionally, the holey optical fiber preform is obtained by a hole opening method, such as a drilling method of performing drilling work on an optical fiber preform, thereby a hole opening is preformed.
The drawing of a holey optical fiber is performed by a method of injecting gas, such as nitrogen gas, into holes of a preform and pressurizing the gas, and a holey optical fiber with a desired hole diameter is obtained by controlling the pressure of the gas to be injected.
On the other hand, it is known that the hole diameter of the holey optical fiber influences the optical properties greatly. For example, it is known that, the smaller the hole diameter of the hole assist optical fiber, the weaker the light confinement effect, whereby the bending loss increases. As such, since the hole diameter is a factor that influences optical properties, it is necessary to maintain the hole diameter of the holey optical fiber at a constant value in the longitudinal direction.
As mentioned above, since the hole diameter of the holey optical fiber influences the optical properties greatly, it is necessary to evaluate the uniformity of the hole diameter in the longitudinal direction. Specifically, it is necessary to guarantee the quality by cutting the holey optical fibers along the longitudinal direction and measuring the hole diameter at the cutting end face and the optical properties.
In this method, since the length of the holey optical fiber is short, manufacture of a long optical cable is difficult, and additionally, this method is insufficient as a quality guarantee method of the optical properties in the longitudinal direction. In order to avoid such a problem, method for non-destructively measuring the hole diameter of the holey optical fiber in the longitudinal direction, and guaranteeing optical properties, such as bending loss, is needed.
As a solution, a method of measuring the hole diameter during a drawing process is proposed in Japanese Patent No. 4084762. This method makes light enter an optical fiber from one end thereof during the drawing process to perform OTDR measurement, and measures the attenuation amount of backscattering light caused by bending the optical fiber, thereby the internal diameter of the hole is estimated.
However, in this method, it is necessary to bend the holey optical fiber with a bending diameter of 10 to 20 mm during the drawing process. Therefore, the holey optical fiber may be locally broke due to a bending stress, and a problem such as a poor productivity occurs. Additionally, in case of a holey optical fiber which has a small bending loss like the hole assist optical fiber, measurement of the attenuation amount of the backscattering light becomes difficult.
Additionally, it is also necessary to know the values in the longitudinal direction of the hole position and hole surface roughness that affect the characteristics of the holey optical fiber.